The present invention relates generally to fiber optic telecommunication systems. More specifically, the present invention relates to a fiber optic receiver assembly for use in a passive optical network that must receive and convert burst mode optical signals from a plurality of sources, wherein each signal being received has a different amplitude.
Telecommunication companies are slowly starting to convert their carrier networks from traditional copper wiring to passive optical network systems as part of Fiber To The Premise (FTTP) and Fiber To The Home (FTTH) communication and content delivery services. For example, a number of telecommunications service providers are now offering fiber optic Internet services and fiber optic television services. These passive optical networks are structured in a manner that is generally similar to the older copper wire networks. As can be seen in FIG. 1, a representative passive optical network can be seen to a plurality of communications lines 2 that extend outwardly to each of the individual service locations 4 (homes or businesses) from a central office 6 (CO) location. The CO 6 in turn serves to control, direct and monitor the transmission and receipt of the signals 8, 10, 12 traveling to and from each of the connected individual service locations 4.
One of the current technical issues being addressed in the implementation of the passive optical networking environment is the fact that there is a large variation in the amplitude of the incoming signals 8, 10, 12 being received at the CO 6 from each of the individual service locations 4. For example, in a FTTH system, one service location 4 may be located 0.5 miles from the central office 6, while another service location 4 may be located 5.0 miles from the central office 6. In each of these service locations 4, the transmitters are essentially the same and therefore transmit their respective data signals 8, 10, 12 to the CO 6 using the same output power level. The difficulty arises as a result of the fact that, since optical signals degrade within a fiber optic cable over distance, the signal received 12 at the CO 6 from the closer service location has an amplitude that is larger than the amplitude of the signal received 10 at the CO 6 from the more distant location. These differences in amplitude become a problem because the passive optical network systems are time division multiplexed (TDM) systems, where the CO 6 receiver is constantly receiving timed bursts 8, 10, 12 in a random pattern from each of the different locations, one after another, with a signal spacing of tens of nano-seconds. As a result, the receivers must be able to quickly and accurately detect and convert all of the incoming signal bursts from the different locations, each having varying amplitudes, into valid data.
In order to deal with this issue, typically as is depicted at FIG. 2, the conversion of the bursts to digital data is done by first generating a reference level to which the input data stream is compared. In such an arrangement, the signal received at the transimpedance amplifier (TIA) is first routed through a signal processor to evaluate the entire signal and to detect the signal amplitude. As a result, a reference level (Vref) is generated by the signal processor for each of the signal bursts received, wherein the reference level is an average power level or average amplitude value based on the signal amplitude within the associated signal burst. It can be appreciated that since each burst is different, they each require that a different reference level be calculated. The actual signal burst is then passed along to the post amplifier along with the calculated reference level for processing.
The problem is that in using the prior art signal burst conversion method described above to generate the required reference level, the characteristics of the signal burst must be measured and processed before it is input to the limiting amplifier. In this arrangement, the signal processor measures the average, or peak-to-peak amplitude of the signal burst and generates a reference level (average power level) associated with signal burst. Since the signal gain of the pre-amplified signal is relatively low it is often difficult to calculate the necessary reference level. Further, in this arrangement, the signal processor and the resultant reference level leave the limiting amplifier outside the control loop thereby subjecting the signal to possibly uncontrolled variations that are introduced due to variations in the limiting amplifier itself. Another obvious issue in this method is the need for a signal processor to measure the amplitude of and generate a reference level for the entirety of each and every burst being received.
There is therefore a need for a receiver that can be used in a passive optical networking environment that eliminates the need to preprocess the entirety of each signal burst received in order to generate a reference level. Further, there is a need for a receiver that generates a reference level after the preamble of the signal burst has been amplified by the limiting amplifier that in turn allows the reference level to then be used for processing the remainder of the signal burst. Still further there is a need for a receiver that includes the limiting amplifier in the reference level calculation thereby accounting for offset voltages and current drift generated by the limiting amplifier itself.